Bookbinding binding machines are commonly used to make books starting from corresponding blocks of signatures (each one of them being formed by folding a printed sheet one or more times for defining corresponding pages of the books). Particularly, in the case of bookbinding sewing machines (or simply sewing machines), the signatures are fed in succession to a sewing station, which sews each signature to the preceding ones of a corresponding book block by means of continuous threads. Typically, the sewing station comprises a movable saddle, which individually receives the signatures when in an open position (moved away from a sewing head being formed by a bank of needles and crochets); the movable saddle is then closed for transporting the signature to the sewing head.
Historically, the sewing machine was born as a stand-alone component, with manual feeding of the signatures (i.e., wherein the signatures were deposited onto the movable saddle by an operator). Later on, a transport system has been connected to the sewing machine for facilitating its feeding (being always manual); typically, the transport system was formed by a fixed saddle; the signatures were manually deposited astride the fixed saddle, which transported them in succession to the sewing station, so that each signature was individually thrown onto the movable saddle (being aligned with the fixed saddle when in the open position). Afterwards, a feeding system has been connected to the sewing machine for automating its feeding; the feeding system extracted the signatures in succession from a hopper, opened them in the middle (separating, by means of suckers, its layers each one defined by a fold of the corresponding sheet), and deposited them astride the fixed saddle. Both the transport system and the feeding system were lacking their own motor, and they were connected mechanically to the sewing machine (being provided with its own motor); the transport system and the feeding system were then driven by the same motor of the sewing machine, so as to work synchronously with the sewing machine (i.e., with its sewing station). More recently; the transport system and the feeding system have been integrated into the sewing machine (in corresponding transport station and feeding station, respective); the transport station and the feeding station have continued to be connected mechanically in a fixed way to the motor of the sewing machine, so as to work always synchronously with the sewing station.
A problem of the known sewing machines is the difficulty of sewing signatures being small (i.e., formed by few pages—for example, 4-8 pages per signature) and/or light (i.e., formed by sheets of thin paper—for example, with a weight in grams of 30-60 g/m2). Indeed, in this case the threads may tear the signatures around corresponding holes being made in the signatures for their sewing. Particularly, the drawback pointed out above occurs rather commonly in case the sheets of the signatures are printed with digital printers—instead of offset printers. Indeed, the digital printers (although more versatile than the offset printers, since they allow varying the pages of the signatures in a simple and fast way without the need of creating any printing plate) are not normally capable of printing sheets of very large size, so that the corresponding signatures have a limited number of pages.
In order to overcome such drawback, small and/or light signatures are normally sewn with threads being less tight; however, this impairs the compactness and the integrity of the book block during the next processing phases (causing the production of books being defective or in any case of lower quality). Moreover, it is possible to use a stagger stitch sewing technique (wherein each needle alternatively cooperates with a next crochet and with a preceding crochet), so as to distribute the threads in a more uniform way along the signatures. However, the stagger stitch sewing technique is relatively complicated and it requires a more complex structure of the sewing head (for transporting each thread from the corresponding needle alternatively to the two adjacent crochets); moreover, this does not allow obtaining satisfactory results all the same in case of signatures formed by very few sheets (and in particular by single sheets being folded only once to define 4 pages—in jargon, a four-page signature) and/or with very thin paper.
Alternatively, it is also possible to provide composite (in jargon, overlapped) signatures to the sewing machine; each overlapped signature is formed by more signatures being simply overlapped one to another (so as to make it stronger). However, the formation of the overlapped signatures is more complex (since it requires additional operations after the folding). Moreover, the opening of each overlapped signature in the sewing machine is problematic, because of its high number of layers to be separated; indeed, the suckers commonly used for this purpose may be insufficient to reach the middle thereof. In such case, it is necessary to form the overlapped signature with a projecting lap, and to provide the sewing machine with a blade that intercepts such lap for opening the overlapped signature. However, this requires non-standard folding operations and complicates the structure of the sewing machine.
Recently, sewing machines have also been proposed that implement both the formation of the signatures and their sewing in a single passage. Particularly, each one of these sewing machines (indicated as combined sewing machines to distinguish them from the above-described sewing machines, indicated as traditional sewing machines) is provided with a gathering device that extracts flat sheets in succession from a hopper, and stacks them for forming groups of sheets corresponding to the signatures; as soon as each group of sheets is completed, it is transferred to a folding device, which folds it so as to obtain the corresponding signature. Each signature thus obtained is then deposited astride the fixed saddle, in order to be transported to the sewing station as usual. In this case, the gathering device and the folding device are driven independently of the transport station and the sewing station (instead always connected between them mechanically in fixed way). An example of combined sewing machine is described in EP-A-0846573 (the entire disclosure if which is herein incorporated by reference), while an example of combined sewing machine being available on the market is KRISTEC by Meccanotecnica S.p.A. (KRISTEC is a registered trademark of Meccanotecnica S.p.A in some countries).
The combined sewing machines allow producing books on request in a very simple way; therefore, they lend themselves particularly to be used for the production of books of limited edition, even of a few units (especially in case the corresponding sheets are provided by digital printers). However, the signatures formed in the combined sewing machines only have a few folds (generally, a single one). Therefore, in case of signatures formed be few sheets (especially if made of thin paper) they suffer the same drawbacks pointed out above for the traditional sewing machines. On the contrary, the formation of signatures of many sheets makes the combined sewing machines very slow (since they require collecting all the sheets of each signature in succession before its folding).